Write the reaction of alkenes and alkynes with o3,kmno4.give eqation
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the opposite of elimination reactions. Recall that an elimination reaction had the following general form:
An addition reaction is simply the reverse of this process:
Clearly, the same substances are present in both cases, so the difference between the reaction that prefers one side (say, the products) over the other side (the reactants) is the conditions under which the reaction occurs: other chemicals that may be present, temperature, and so on.
For alkynes, the general elimination reaction is shown below, followed by the corresponding addition reaction.
Hydrogenation
Alkenes and alkynes are called unsaturated hydrocarbons because, as the name indicates, the carbon atoms are not "saturated" with hydrogens, owing to the presence of double or triple carbon-carbon bonds. As a result, these substances tend to be more reactive at the site of the multiple bond than a saturated hydrocarbon (an alkane or cycloalkane), which cannot accept any additional atoms. In the presence of heat, saturated hydrocarbons can eliminate hydrogen gas to form unsaturated hydrocarbons (alkenes and alkynes). The reverse process, hydrogenation, can be performed in the presence of a catalyst like metallic platinum. Hydrogen gas reacts with the metal surface, breaking the hydrogen-hydrogen bond to form weaker metal-hydrogen bonds. An alkene or alkyne can then react with the metal in a similar manner, then form stronger bonds with two or more hydrogen atoms. This process is illustrated below for a generic alkene.
The alkene forms similar weak carbon-metal bonds at the site of the double bond.
The weak metal bonds are then replaced by stronger carbon-hydrogen bonds, and the alkene (or alkyne, by a similar process) becomes saturated.
The process of hydrogenation does not occur such that both carbons are saturated simultaneously. Nevertheless, hydrogenation, as the diagram indicates, results in addition of hydrogen atoms to the same "side" of the alkene. (This result is more apparent in the case of cycloalkenes, however.) In the case of alkynes, the hydrogenation process can produce alkenes (rather than completely saturated alkanes) through the use of a so-called Lindlar catalyst, like Lindlar palladium.
Practice Problem: The following molecule is combined with hydrogen gas in the presence of Lindlar palladium. What is the product of the reaction?
Solution: A Lindlar catalyst only hydrogenates alkynes. Thus, the product molecule is a diene (hydrocarbon with two double bonds), which is shown below. The IUPAC name is 7-methyl-1,6-octadiene.
Electrophilic Addition
As the name implies, an electrophile is an "electron-loving" or "electron-seeking" compound that can act as a Lewis acid (electron pair acceptor). Electrophiles can react with the double bond of an alkene, resulting in an electrophilic addition reaction. Consider the simple case of ethylene (ethene) reacting with a hydrogen halide, HX (where X is the halogen). Hydrogen halides are dipolar molecules, and the proton side can approach the 2p orbital of ethylene and accept an electron pair (i.e., act as a Lewis base). The ensuing reaction is shown below. First, the rate-determining step involves creation of a halide ion and a carbocation.
The carbocation, which readily accepts an electron pair, then reacts with the halide ion to form an alkyl halide.
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Overall, then, the reaction is summarized below.
In the case of addition to more complicated alkenes, Markovnikov's rule states that the proton adds to the carbon with the most hydrogen atoms attached, and the halogen adds to the carbon with the fewest hydrogen atoms attached. Thus, for instance, addition of HCl to 2-methyl-2-pentene yields 2-methyl-2-chloropentane.
Similarly, alkenes can be converted into alcohols by a reverse process involving concentrated sulfuric acid. This process is called hyd
An addition reaction is simply the reverse of this process:
Clearly, the same substances are present in both cases, so the difference between the reaction that prefers one side (say, the products) over the other side (the reactants) is the conditions under which the reaction occurs: other chemicals that may be present, temperature, and so on.
For alkynes, the general elimination reaction is shown below, followed by the corresponding addition reaction.
Hydrogenation
Alkenes and alkynes are called unsaturated hydrocarbons because, as the name indicates, the carbon atoms are not "saturated" with hydrogens, owing to the presence of double or triple carbon-carbon bonds. As a result, these substances tend to be more reactive at the site of the multiple bond than a saturated hydrocarbon (an alkane or cycloalkane), which cannot accept any additional atoms. In the presence of heat, saturated hydrocarbons can eliminate hydrogen gas to form unsaturated hydrocarbons (alkenes and alkynes). The reverse process, hydrogenation, can be performed in the presence of a catalyst like metallic platinum. Hydrogen gas reacts with the metal surface, breaking the hydrogen-hydrogen bond to form weaker metal-hydrogen bonds. An alkene or alkyne can then react with the metal in a similar manner, then form stronger bonds with two or more hydrogen atoms. This process is illustrated below for a generic alkene.
The alkene forms similar weak carbon-metal bonds at the site of the double bond.
The weak metal bonds are then replaced by stronger carbon-hydrogen bonds, and the alkene (or alkyne, by a similar process) becomes saturated.
The process of hydrogenation does not occur such that both carbons are saturated simultaneously. Nevertheless, hydrogenation, as the diagram indicates, results in addition of hydrogen atoms to the same "side" of the alkene. (This result is more apparent in the case of cycloalkenes, however.) In the case of alkynes, the hydrogenation process can produce alkenes (rather than completely saturated alkanes) through the use of a so-called Lindlar catalyst, like Lindlar palladium.
Practice Problem: The following molecule is combined with hydrogen gas in the presence of Lindlar palladium. What is the product of the reaction?
Solution: A Lindlar catalyst only hydrogenates alkynes. Thus, the product molecule is a diene (hydrocarbon with two double bonds), which is shown below. The IUPAC name is 7-methyl-1,6-octadiene.
Electrophilic Addition
As the name implies, an electrophile is an "electron-loving" or "electron-seeking" compound that can act as a Lewis acid (electron pair acceptor). Electrophiles can react with the double bond of an alkene, resulting in an electrophilic addition reaction. Consider the simple case of ethylene (ethene) reacting with a hydrogen halide, HX (where X is the halogen). Hydrogen halides are dipolar molecules, and the proton side can approach the 2p orbital of ethylene and accept an electron pair (i.e., act as a Lewis base). The ensuing reaction is shown below. First, the rate-determining step involves creation of a halide ion and a carbocation.
The carbocation, which readily accepts an electron pair, then reacts with the halide ion to form an alkyl halide.
Want to learn more? Take an online course in Organic Chemistry.
Overall, then, the reaction is summarized below.
In the case of addition to more complicated alkenes, Markovnikov's rule states that the proton adds to the carbon with the most hydrogen atoms attached, and the halogen adds to the carbon with the fewest hydrogen atoms attached. Thus, for instance, addition of HCl to 2-methyl-2-pentene yields 2-methyl-2-chloropentane.
Similarly, alkenes can be converted into alcohols by a reverse process involving concentrated sulfuric acid. This process is called hyd
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